1. Technical Field
The present invention relates generally to photolithography technology used for wafer fabrication, and more particularly, to a method for pattern overlay measurement using a multiplex filter with several color filters in the exposure process.
2. Discussion of the Related Art
Wafer fabrication encompasses many manufacturing processes that create the semiconductor device, i.e., integrated circuits, in and on a wafer. Circuit formation in and on the wafer requires much layering and patterning. With layering, thin layers of different materials are grown on or added to a surface of the wafer. After the wafer is layered, selected portions of the layer are removed, leaving a pattern remaining on the wafer. This patterning process is called photolithography.
In photolithography, a specific pattern preformed on a mask is transferred to a layer on the wafer surface by imaging the pattern onto a photoresist which overlies the layer. This imaging process is referred to as the exposure process.
The initial step of the exposure process is a precise alignment of the wafer with the mask. Overlay measurement is routinely performed to verify the proper alignment. For the overlay measurement, overlay marks are preformed in the layer on the wafer and in the mask. The overlay measurement optically measures the relative positions of the overlay marks in different layers on the wafer and on the mask.
FIG. 1 schematically shows a known apparatus 100 for performing the overlay measurement. Referring to FIG. 1, the overlay measurement apparatus 100 includes a wafer table 30 on which the wafer 40 is mounted, a lens 10 used to produce an image of the overlay mark formed on the wafer 40, and a multiplex filter 20 used to select wavelengths of light rays passing through the lens 10 according to surface conditions of the wafer 40.
The multiplex filter 20 is schematically shown in a plan view of FIG. 2. As shown in FIG. 2, the multiplex filter 20 is composed of a filter body 21 having a disk-like shape, and six color filters 23a, 23b, 23c, 23d, 23e and 23f having different colors, such as red, green, orange, blue, yellow and white. For example, these color filters are arranged in order of the green filter 23b, the orange filter 23c, the blue filter 23d, the yellow filter 23e and the white filter 23f, in a clockwise direction, starting from the red filter 23a. 
During measurement of the overlay marks, the ability to perceive the overlay marks may vary depending on surface conditions of the layers in which the overlay marks are formed. Accordingly, to perceive the overlay marks well, light rays with a specific wavelength are required. Each color filter allows passing of light rays having a desired wavelength.
FIG. 3 illustrates, in a flow diagram, a conventional method 50 for overlay measurement during the exposure process. Referring to FIG. 3 together with FIGS. 1 and 2, in a first step 51, a human operator selects a specific color filter 23 suitable for measuring the overlay marks on the wafer 40 and then fixes the multiplex filter 20 to the apparatus 100.
A next step 52 is to attempt to perceive the overlay marks on the wafer 40 through the lens 10. If the operator is able to perceive the overlay marks, the operator measures the overlay marks on the wafer 40 through the lens 10 (step 53). Next, the operator analyzes measured values of the overlay marks (step 54), and calculates analyzed results (step 55). Calculated data is then fed into the overlay measurement apparatus 100 (step 56). If the operator is not able to perceive the overlay marks in the step 52, the operator acknowledges errors in measuring (step 57) and stops the exposure process (step 58).
Such errors in measuring may be caused by failing to perceive the overlay marks due to irregular wafer surfaces. As the wafer diameter increases to, for example, twelve inches, a difference in thickness between central regions and peripheral regions of the wafer becomes greater, especially, after a chemical mechanical polishing (CMP) process. As a result, the overlay marks may differ in height as well, depending on surface conditions of the regions where the overlay marks are formed. The fixed color filter cannot be used to perceive all overlay marks having different heights, which results in perception failure of the overlay marks and resultant measuring errors.